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  • 1990-1994  (582)
  • 1992  (582)
  • Computational Chemistry and Molecular Modeling  (582)
  • Nuclear reactions
  • 1
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 41 (1992), S. 7-14 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 41 (1992), S. 53-75 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The geometric phase of quantum mechanics is introduced, and its physical effects are then studied in the context of molecular physics. By performing the most general Born-Oppenheimer procedure, we show how gauge groups appear in the study of molecules. This method is then applied to the doubly degenerate Λ-levels of a diatomic molecule. The resulting dynamics for the slow angular motion of the dumbbell is equivalent to that of a Dirac monopole.
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  • 3
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 41 (1992), S. 147-151 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Explicit expressions for the eigenvalues of the class sums [(p)(1)n-p]n, p = 2, 3,…,14, of the symmetric group Sn are presented. Partial results are given for the eigenvalues corresponding to arbitrary p.
    Additional Material: 1 Tab.
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  • 4
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 41 (1992) 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 41 (1992), S. 257-271 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In the framework of our study of the changes of the electric polarizability upon substituting heteroatomic linkages in conjugated hydrocarbon backbones, we have been led to examine the relative stability of eight compounds isoelectronic to the all-trans octatetraene using the Cox and Pilcher concept of stabilization energy (SE), SE = ΔHa - ΣNABEAB, where ΔHa is the heat of atomization of the species under consideration and the EAB's are bond-energy terms. Full geometry optimizations at the 4-31G level have been performed to obtain the equilibrium geometries needed to deduce reliable semiempirical heats of formation from appropriate isodesmic processes. Some of the compounds containing the — CH=N— and — N=N — groups have electric polarizabilities and stabilization energies comparable to octatetrene.
    Additional Material: 1 Ill.
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  • 6
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 41 (1992), S. 311-325 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We derived the necessary conditions to which the vector coupling coefficients (VCC) amn(u) and bmn(u) describing atomic L,S-multiplets of the configurations dN (1 ≤ N ≤ 9), should satisfy. Special attention is paid to the states of non-Roothaan type for which VCC depend on the choice of degenerate d-orbitals basis set determined within the accuracy up to an orthogonal transformation u. It is shown that for such states the direct sum of matrices ‖amn(u)‖ and ‖bmn(u)‖ must be the non-symmetric matrix. Obtained VCC were used for the ab initio calculations (basis set (14s9p5d)/[8s4p2d] from [15]) on first-row transition atoms (from Sc to Cu) to compare to similar calculations [16], in which the Peterson's VCC have been used, and with calculations [15] carried out by the atomic SCF program [4] as well.
    Additional Material: 4 Tab.
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  • 7
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 41 (1992), S. 339-357 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The modified and extended version of the Neumann expansion of the interrelectronic distance function riju for u = -1, 0, 1, 2, using the set of orthogonal polynomials normalized to unity, is presented. This expansion has been utilized to obtain analytical expressions for evaluating two-center two- and three-electron integrals in the Slater orbital basis occurring if variational correlated functions are used.
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  • 8
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 41 (1992), S. 385-385 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 41 (1992), S. 517-524 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The density of states is calculated for a random distribution of donor-pairs of hydrogenlike impurities in three- and two-dimensional systems. Recent investigations of the hydrogen molecule in the alternant-molecular-orbital approximation are here extended. We found that the lowest excited state 1Σu (i.e., H+H-), which is optically connected to the ground state, plays a relevant role in the absorption spectra of semiconductor systems.
    Additional Material: 8 Ill.
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  • 10
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 41 (1992), S. 1-2 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 11
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 42 (1992), S. 339-347 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The properties of the n-electron wave functions with symplectic symmetries have been discussed. These functions are built up with two kinds of geminals. The well-known antisymmetrized geminal power (AGP) function is only a special case of them. The first- and second-order reduced density matrices and the natural expansions of these functions have been given, and the uniform expressions for system energies in the states corresponding to these functions have been deduced. It has been shown that the special case is probably another consistent ground state for the particle-hole propegator or random-phase approximation (RPA) besides the AGP function.
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  • 12
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 42 (1992), S. 403-409 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In this paper, the reactions of HNCO or CH3NCO with methanol have been studied using the ab initio MO method. The geometries of the reactants and products have been optimized by the energy gradient method. The calculated results are in accordance with experiment. Using Powell's method, by the minimization of the Euclidean norm σ of the gradient, the structures of the transition state (TS) for the two reactions were obtained. The structures have been further confirmed as TS by finding that there is one and only one negative eigenvalue for their force constant matrix. The imaginary vibration mode corresponding to the TS was also discussed. The calculated activation barriers of these two reactions are 96.02 and 95.13 kJ/mol, respectively. It can be concluded that the alcoholysis reaction of isocyanate is a nucleophilic addition reaction with methanol as an electronic donor and isocyanate as an acceptor and that the hydroxyl hydrogen of CH3OH plays an important role in the reaction.
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  • 13
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 41 (1992), S. 381-382 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Additional Material: 1 Tab.
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  • 14
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 41 (1992), S. 421-435 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The recently developed inclusion of the spin-orbit interaction into the quasi-relativistic version of the MSXα method has been applied to the ThX4:Pa4+ (X ≡ Cl, Br) doped system. A new interpretation of the optical spectrum is given.
    Additional Material: 4 Ill.
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  • 15
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 41 (1992), S. 437-442 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Given a function space spanned by a basis {ƒi}, we are interested in finding another basis {gi} for which the overlaps (gi | gi) assume arbitrarily prefixed values in a subset B of the full set of the pairs of indices (i, j). The other overlaps are let free. We show how it is possible to perform this linear transformation ƒi → gi minimizing the “distortion” J = Σi(gi - ƒi | gi - ƒi).
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  • 16
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 41 (1992), S. 461-474 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Phonon-assisted interchain hopping of negatively charged solitons in polyacetylene has been studied using a local chemical reaction model CH3- + CH4 → CH4 + CH3-. Quantum chemical characteristics of the electron transfer process have been analyzed in terms of the dynamic electron density and the mutual polarization moment. The CH stretching vibrational motion of CH4, which is a local model of the sp3 defect, has been found to play a significant role for the electron transfer. The excitation of the corresponding vibrational mode of the sp3 defect would promote the interchain hopping of the charged soliton. The electron transfer process has also been studied in terms of the “regional” density functional theory. It has been shown that the driving force of the electron transfer is represented by the regional chemical potentials.
    Additional Material: 6 Ill.
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  • 17
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The recently proposed ab initio method for calculation on many-electron molecular systems with the approximation of the inactive part of a molecule by a frozen molecular fragment was tested further in a case of the dissociation reaction of the C—F bond in n-fluoropropane. Results from the Hartree-Fock, multiple reference double-excitation configuration-interaction and second-order Møller-Plesset methods are presented. The reproduction of potential energy surfaces as well as the reproduction of electron density distribution are in excellent agreement with extended basis-set calculations. Different choices of fragments to be frozen have been examined.
    Additional Material: 3 Ill.
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  • 18
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 41 (1992), S. 591-598 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The addition reactions between HO2 and propene leading to the radical intermediates CH3CHCH2OOH and CH3CHOHCH2 have been studied by ab initio molecular orbital calculations using a 6-31G* basis set and including electron correlation through fourth-order Møller-Plesset calculations. The intermediates are predicted to have energies of about 5 kcal/mol below the total reactant energies, the complex resulting from the HO2 attack on the central carbon of propene being slightly preferred. The activation energies for the addition to the terminal carbon and the central carbon are predicted to be 8.5 and 8.0 kcal/mol, respectively, at the highest level of calculation [MP4(SDTQ)] with corrections for spin contamination. Spin contamination corrections are found to be very important in the calculation of these values. Referring to previous calculations at the same level for the addition of HO2 to ethylene [12], we assume that the addition step is the rate-determining one in the reaction leading to HO and propene oxide. The observed activation energy for this reaction, 14.2 kcal/mol [2], is significantly higher than the predicted one for the addition step. The discrepancy found, 6.2 kcal/mol, is virtually the same as the one encountered in the ethylene case, 6.6 kcal/mol [12]. The barrier to intramolecular hydrogen migration leading to the intermediate radical CH2CH2CH2OOH is found to be 42.6 kcal/mol at the highest level of calculation. Spin contaminiation corrections are not important for this energy.
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  • 19
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 41 (1992), S. 729-731 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The second-order Rayleigh-Schrödinger energy correction to the Born-Oppenheimer potential energy due to the spin-orbit interaction can be expressed as a linear response function evaluated at zero frequency. We have calculated this energy contribution to the Cr2 singlet ground state X1 Σ+g potential energy function for a multiconfiguration self-consistent field (MCSCF) wave function. The calculations show that the effect of spin-orbit interaction is small and of the same magnitude for the whole potential energy curve.
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  • 20
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 41 (1992), S. 749-754 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A commercial computer algebra program, Mathematica, is used to generate the C matrix that characterizes our implementation of the Löwdin α-function method as applied to Slater-type orbitals. An example of a two-center overlap integral is done to show how the arbitrary precision capability of Mathematica can overcome severe cancellation errors encountered with programming in FORTRAN. This strategy is capable of being generalized to other multicenter molecular integrals. Mathematica programs are included.
    Additional Material: 3 Tab.
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  • 21
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 42 (1992), S. 1237-1250 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The possiblity of creating a useful model for the electrostatic potential around the water molecule is examined from two different starting points. It is suggested that the simplest and most accurate model will have three point charges on the nuclei together with a diffuse Gaussian charge centered near the nuclear center of the charge. This will represent the lone-pair cloud, include the penetration effects, and improve the representation of the electron density. The alternative use of atomic dipoles is explored but seen to be less effective.
    Additional Material: 3 Ill.
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  • 22
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 41 (1992), S. 793-810 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An ab initio electronic structure study is presented of hydrogen-hydrogen interactions in an electronic environment perturbed by the presence of palladium atom clusters. In particular, we investigated changes in the interatomic potential when the atomic centers are trapped inside an fcc palladium octahedral hole and when they are separated from each other by a (111) plane of palladium atoms. The (111) plane was modeled with a cluster of three palladium atoms. Self-consistent field (SCF) level calculations were performed, and palladium atom pseudopotentials were employed to make the systems studied computationally tractable. For pairs of atoms placed within the octahedral hole, various lines of approach were considered [along the (100), (110), and (111) directions]. Lattice deformations and electronic excitations were examined for their effect on the interatomic potential.
    Additional Material: 13 Ill.
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  • 23
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 42 (1992), S. 5-33 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Silicon is taken as a test system for assessing present-day feasibility of calculations for crystalline solids of near-Hartree-Fock quality. The calculations have been performed using CRYSTAL, an ab initio Hartree-Fock crystalline-orbital LCAO program for periodic systems. The influence of the computational parameters that control the truncation of infinite sums on the results has been investigated; it is shown that a reasonable accuracy (numerical errors on total energy per atom below 10-3 a.u.) can be obtained while keeping the computational burden within manageable limits. The effect on the results of basis-set size and quality is discussed. A number of basis sets have been tested, from minimal to relatively extended sets (28 atomic orbitals per atom). The quality of the wave function has been checked using variational criteria and also through a comparison with experimental data, such as equilibrium geometry, bulk modulus, electron charge density, and electron momentum distribution. For the latter quantities, which are a measure of the accuracy of the one-electron density matrix, the best basis sets provide agreement with experiment that is almost within the experimental error. The correlation energy has been estimated using nonlocal density functionals, based on the one-electron density matrix: After this correction, the atomization energy agrees with experiment to within 2%. The generalization of the above analysis to other crystals is briefly discussed.
    Additional Material: 4 Ill.
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  • 24
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 42 (1992), S. 1499-1514 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The results of the Monte Carlo Metropolis simulation of water structure and of the hydration of nucleic acid fragments, complementary base pairs and mispairs, base pair stacks, and duplex fragments have been summarized. Systematic investigations suggest some general conclusions: (1) the hydration shell structure of the major and minor grooves of the duplex depends significantly on DNA conformation (or stack configuration) and nucleotide sequence, while global hydration characteristics (average energy, the number of water-water and water-base H-bonds) are only slightly dependent on these factors, (2) hydration economy takes place in the B-A transition due to an increase of the number of water molecules forming hydrogen bonds with two or more atoms of bases (water bridging), and (3) the hydration of the duplex could contribute to nucleic acid functioning via water-bridged mispair formation and stabilization of specific conformations.
    Additional Material: 3 Ill.
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  • 25
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 42 (1992), S. 1479-1489 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Time-resolved fluorescence of single tryptophan proteins have demonstrated the complexity of protein dynamic and protein structure. In particular, for some single tryptophan proteins, their fluorescence decay is best described by a distribution of fluorescence lifetimes rather than one or two lifetimes. Such results have provided further confirmation that the protein system is one which fluctuates between a hierarchy of many conformational substates. With this scenario as a theoretical framework, the correlations between protein dynamic and structure are investigated by studying the time-resolved fluorescence and anisotropy decay of holo and apo human superoxide dismutase (HSOD) at different denaturant concentrations. As a function of guanidine hydrochloride (GdHCl), the width of the fluorescence lifetime distribution of HSOD displays a maximum which is not coincident with the fully denatured form of HSOD at 6.5M GdHCl. Furthermore, the width of the fluorescence lifetime distribution for the fully denatured forms of holo and apo HSOD is greater than that of the native forms.
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  • 26
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Static polarizability and second hyperpolarizability have been calculated for a number of small molecules—CO2, OCS, CS2, C2H2, C2H6, C3H8, cyclo-C3H6, C3H4, C3H6, SiH4, Si2H6—in the framework of the coupled-perturbed Hartree-Fock (CPHF) theory. The linear and nonlinear coefficients have been calculated with standard Gaussian basis sets and 3-21G bases moderately enlarged with diffuse functions. It is shown that the parallel component of the polarizability saturates rapidly, which suggests that a 3-21G basis containing s and p diffuse functions is sufficient to reproduce αzz. For the αxx and αyy components, a 3-21G basis with s, p, and d diffuse functions is required. In general, the concordance between α computed with this basis set and the experimental static polarizability is at least of the order of 80%. On the contrary, the computation of the second hyperpolarizability with the same basis set for CO2, CS2, and C2H2 gives values that are 30% too low, compared to the experimental value. Better results are observed for ethane, propane, and cyclopropane for which the error is lower than 50%. The better agreement observed for the saturated compounds can probably be explained by their saturated character.
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  • 27
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    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 42 (1992), S. 1625-1632 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Possible applications of computational molecular-engineering approaches to the design of bistable conformational systems, to be potentially employed in molecular electronics, have been analyzed in isomeric-thermodynamic terms. Constructing such systems led to (conflicting) requirements, viz. an easy interconversion of the two structures (the isomerization equilibrium constant close to unity). At the same time there is a need for both isomers to be sufficiently stable and mutually different (enthalpy-entropy compensation can still ensure the equilibrium-constant requirement). Moreover, the equilibrium criteria have to be necessarily combined with considerations of kinetics. The design problems are illustrated on two model systems: Si6H6 isomeric species and deuterium- and hydrogen-bonded water dimers, HOD · OHD and DOH · OHD, respectively.
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  • 28
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 42 (1992), S. 1651-1657 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The low-lying allowed electronic transition energies of four alternative structures to the C60 cage molecule are examined. The bond turning transformation of Stone and Wales is used to generate new C60 structures with the symmetries C2v, D2h, and D6h. We have found that the C60 is indeed a truncated icosahedron as originally proposed, but the production of other alternative structures in specific experimental conditions is not excluded.
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  • 29
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 42 (1992), S. 459-474 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An analysis of molecular shape has been performed along an ab initio reaction path of the PPO → OPP isomerization. In a first approximation, we have used the fused-sphere (van der Waals) model to represent shape. The results show that there is no one-to-one correspondence between the species defined in terms of shape and those based on an energy criterion. In contrast to the cases of most other isomerizations in triatomic molecules (e.g., the HCN isomerization), for the PPO isomerization there exists a stable intermediate. For the elementary reaction from the linear PPO (C∞v, reactant) to the cyclic P2O (C2v, intermediate), we found no distinct molecular shape type for the transition structure (between the reactant and the intermediate). In terms of shape, the nuclear configuration of the transition structure is more similar to that of the intermediate than that of the reactant. Using the PPO → OPP reaction as an example, in this work we discuss the implications of this result regarding possible measures of shape similarity and shape stability along reaction paths. We have performed a molecular-shape analysis of the electron density contour surfaces to test the reliability of the fused-sphere approximation. A comparison between the van der Waals and electron isodensity surface models at critical points shows that the former model can be qualitatively correct for molecular-shape descriptions along some reaction paths.
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  • 30
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 43 (1992), S. 5-6 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 31
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 43 (1992), S. 83-90 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A simple one-parameter formula: is introduced for the description of the dispersion in the second dynamic hyperpolarizability (γ), where ω = ω1 + ω2 + ω3 and ωL2 = ωσ2 + ω12 + ω22 + ω32. This formula is appropriate for both atoms and the macroscopic hyperpolarizabilities of molecules (as defined with respect to laboratory axes). Its suitability is tested for helium for five different nonlinear optical processes (including, for the first time, the ac Kerr effect). A comparison is made between the values of A found by fitting ab initio γ values and those derived from a formula proposed by Hellwarth et al.
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  • 32
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    International Journal of Quantum Chemistry 42 (1992), S. 663-683 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Equations of motion on the levels of classical mechanics, hydrodynamics, and wave mechanics are analyzed and existing connections are shown. The manifestation of particle and wave aspects on the different levels are investigated. Dissipative frictional forces are included and a nonlinear wave mechanical equation for these systems can be found. It follows the discussion of an alternative to the time-dependent Schrödinger equation (SE) in order to obtain the dynamical information on particle and wave aspects in a different form. This can be achieved by means of a Newtonian equation for a complex variable in connection with a conservation law for a nonclassical angular momentum-type quantity. With the help of this complex variable it is also possible to develop a Hamiltonian formalism for the wave aspect contained in the SE, the fluctuation, or uncertainty of position and momentum. In this case the Hamiltonian function is equivalent to the difference between the mean value of the Hamiltonian operator and the classical Hamiltonian function. Possible extensions and applications are mentioned.
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  • 33
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    International Journal of Quantum Chemistry 43 (1992) 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 34
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    International Journal of Quantum Chemistry 43 (1992), S. 209-214 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We generalize the usual complex virial theorem using a procedure of complex scaling for only some of the coordinates of the system. This result can also be considered as an extension of the virial theorem obtained in a previous work using real scaling for a subset of coordinates.
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  • 35
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    International Journal of Quantum Chemistry 43 (1992), S. 251-258 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The variation-iteration method of Svartholm has been applied to the momentum-space Schrödinger equation for one-dimensional two-electron systems. The first and second iteration momentum-space wave functions have been evaluated in analytical forms. The momentum representation of the exact Hartree-Fock ground-state wave function is chosen as the initial function. The influence of electron correlation on the distribution of momentum-space probability density has been studied. It is shown for the one-dimensional McWeeny-Coulson problem that the numerical value of the ground-state energy of the one-dimensional two-electron atom is between the minimum energy values ε1/2 and ε1.
    Additional Material: 3 Ill.
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  • 36
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    International Journal of Quantum Chemistry 43 (1992), S. 281-300 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A simple valence-bond approach is considered for treating extended systems of equivalent atoms. In this approach, a limited nonorthogonal basis set of atomic orbitals is utilized and only nearest-neighbor interactions are supposed. By exploiting then the translational invariance of the system, expressions for the matrix elements between antisymmetrized products of such nonorthogonal orbitals are derived in an algebraic form suitable for the numerical implementation on a computer. The one-band linear case is explicitly considered. Some possible applications of the proposed approach and its connections with model Hamiltonians such as the Hubbard and the Heisemberg ones are also discussed.
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  • 37
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    International Journal of Quantum Chemistry 43 (1992) 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 38
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The electron-vibrational problem of the general nonadiabatic molecular systems has been solved by means of the quasi-particle transformations. The SCF ab initio solution of the nonadiabatic fermion Hamiltonian yields stabilization of the electronic ground-state energy due to electron-phonon interaction and it also gives the corrections to the one- and two-particle terms. Two two-particle correction yields effective attractive electron-electron interaction, but in the form different from Frölich's effective electron-electron interaction term. In contrast to the standard electron-phonon Hamiltonian of solid-state physics that does not take into account the possible effects of nonadiabaticity of a system, the presented nonadiabatic theory yields also one-particle corrections. The presence of this term in the Hamiltonian might play a crucial role in the theory of superconductivity since the superconductors are nonadiabatic systems.
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  • 39
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    International Journal of Quantum Chemistry 43 (1992), S. 437-437 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 40
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    International Journal of Quantum Chemistry 42 (1992), S. 1553-1561 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Individual hydration water molecules in aqueous protein solutions have been observed using experimental schemes for homonuclear two-dimensional and heteronuclear three-dimensional NMR experiments in H2O solution, which do not require suppression of the solvent line by presaturation. In these experiments, the location of the hydration waters is determined from their nuclear Overhauser effects (NOEs) with individual hydrogen atoms of distinct amino acid residues. In the basic pancreatic trypsin inhibitor (BPTI), four internal water molecules that had been reported in three different crystal forms were also found to be in the same locations in the solution structure, with lifetimes with respect to exchange of the water protons in excess of 0.3 ns. Additional NOEs with polypeptide protons located on the protein surface may involve either hydration water molecules or hydroxyl protons of amino acid side chains. Their total number is small compared to the number of NOEs expected from the hydration water molecules identified in the crystal structures of BPTI.
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  • 41
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    International Journal of Quantum Chemistry 43 (1992), S. 783-800 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We report a comparison of theoretical and experimental proton affinities at nitrogen and oxygen sites within a series of small molecules. The calculated proton affinities are determined using the semiempirical methods AM1, MNDO, and PM3; the ab initio Hartree-Fock method at the following basis levels: 3-21G//3-21G, 3-21+G//3-21G, 6-31G*//6-31G*, and 6-31+G(d, p)//6-31G*; and Møller-Plesset perturbation calculations: MP2/6-31G*//6-31G*, MP3/6-31G*//6-31G*, MP2/6-31G+(d, p)//6-31G*, MP3/6-31G+(d, p)//6-31G*, and MP4(SDTQ)/6-31G+G(d, p)//6-31G*. The semiempirical methods have more nonsystematic scatter from the experimental values, compared to even the minimal 3-21G level ab initio calculations. The thermodynamically corrected 6-31G*//6-31G* proton affinities provide acceptable results compared to experiment, and we see no significant improvement over 6-31G*//6-31G* in the proton affinities with any of the higher-level calculations. © 1992 John Wiley & Sons, Inc.
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  • 42
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    International Journal of Quantum Chemistry 42 (1992), S. 1641-1649 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The simplest nonrelativistic density functional theory, namely, that of Thomas, Fermi, and Dirac, is used to study the effect of exchange on the equilibrium bond lengths of heavy tetrahedral molecules XH4. In particular, the limiting bond length as the central atom X becomes infinitely heavy is shown to be reduced by exchange by some 0.34 Å. Comparison with experiment shows that the main features of the bond-length variation through the series CH4—PbH4 are reflected by the Thomas-Fermi-Dirac predictions, though the bond lengths remain too large for finite atomic number Z of the central atom. Therefore, a semiempirical correction is proposed, which, however, tends to zero as Z tends to infinity.
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  • 43
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    International Journal of Quantum Chemistry 44 (1992), S. 45-57 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The possible combination of two methods for the calculation of multicenter two-electron integrals using STO and B function basis sets is discussed. The first method (Method I), which is of approximate nature, is based on a simplified version of the so-called Σ-factorization method [A. W. Niukkanen and L. A. Gribov, Theor. Chim. Acta 62, 443 (1983)], where the radial part of the two-center one-electron density ρab(r) is represented as a sum of two radial functions fa(r) and fb(r), placed on two different centers a and b. After such a transformation, the calculation of the two-electron integrals boils down to the calculation of some type of Coulomb integrals. The second method (Method II), which calculates each integral separately to a given accuracy, is based on Mobius-type quadrature used for a three-dimensional integral representation for the two-electron integral of B functions [E. O. Steinborn and H. H. H. Homeier, Int. J. Quantum Chem. Symp. 24, 349 (1990)]. In Method I, the choice of the radial functions placed on the different centers has an essential influence on the final value of the multicenter integrals. In the present study of Method I, a rather simple approximation of the radial part was made that reproduced the qualitative behavior of the molecular integral curves as function of the geometry rather well. At the present state of development, Method I produces fast order-of-magnitude estimates that are useful for screening purposes, i.e., to decide which integrals have to be evaluated more accurately by other methods like Method II. Method II reproduces data given in the literature [R. M. Pitzer and D. P. Merrifield, J. Chem. Phys. 52, 4782 (1970)] correctly. © 1992 John Wiley & Sons, Inc.
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  • 44
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    International Journal of Quantum Chemistry 44 (1992) 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 45
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    International Journal of Quantum Chemistry 44 (1992), S. 81-89 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Closely related structures, like esters and lactones, have vastly different physical properties. This is apparently due to differences in the intermolecular interactions. The intermolecular interactions of methyl acetate, β-propiolactone, ethyl acetate, and γ-butyrolactone have been studied using the AM1 semiempirical method. Some of the “arranged clusters” were also compared to possible covalently bound trimers and tetramers of β-propiolactone and γ-butyrolactone. © 1992 John Wiley & Sons, Inc.
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  • 46
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    International Journal of Quantum Chemistry 44 (1992), S. 113-122 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Local density functional theory (DFT-LDA) has been explored as a tool for obtaining the molecular electrostatic potential V(r), using the code DMol. We have presented and discussed DFT-LDA electrostatic potentials for a representative series of molecules: ethylene, benzene, formamide, cytosine, and 2,3,7,8-tetrachlorodibenzo-p-dioxin. V(r) results obtained with a double numerical plus polarization (DNP) basis set show the key features that are characteristic of the ab initio potentials of these compounds and suggest that this is a useful approach, especially for large molecules that are difficult to study by ab initio methods.
    Additional Material: 5 Ill.
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  • 47
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    International Journal of Quantum Chemistry 44 (1992), S. 141-163 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Semiempirical methods were utilized in the computation of a fully optimized structure of bilirubin. Bond lengths and bond angles obtained using either AM1 or PM3 calculations showed excellent agreement with those obtained by X-ray diffraction. This indicated that molecular orbital methods satisfactory reproduced the complex conjugation found in bilirubin. Dihedral angles of the crucial “hinge” and the dihedral angles of the propionic acid side chains agreed well with those found by X-ray diffraction. Calculated hydrogen- bond parameters (distance and angles) showed substantial differences from experimental values, probably due to inherent weakness in the parameterization of the molecular orbital techniques. Conformational studies were carried out using AM1 by rotating the C9—C10 bond in 5° increments showed that the most stable structure exhibited a minimum at about 125° and exhibited a structure similar to those postulated from X-ray and NMR experiments. The hydrogen bonds showed remarkable tenacity during rotation of the C9—C10 bond and resisted breaking until the molecule was under extreme strain. © 1992 John Wiley & Sons, Inc.
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  • 48
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    International Journal of Quantum Chemistry 44 (1992), S. 181-202 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Semiempirical calculations of the α-2 receptor antagonist, Idazoxan, its methoxy derivative, and their respective cations and anions revealed the most stable conformers were significantly affected by electrostatic interactions. These studies showed the importance of the intramolecular electrostatic interactions between the oxygen of the benzodioxane ring and the nitrogens of the imidazoline ring in determining the global minima of these molecules. The NH bond of the imidazoline ring was attracted to the dioxane oxygen and caused the imidazoline ring to rotate to a position allowing close proximity of these groups. Favorable interactions between the oxygens and the imidazoline stabilized the axial conformer, whereas unfavorable interactions increased the stability of the equatorial isomer. AM1 and PM3 calculations sometimes predicted different global minimum conformers, indicating the need for caution in reliance on either method, particularly when hydrogen bonds play an important role in determining the structure. © 1992 John Wiley & Sons, Inc.
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  • 49
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    International Journal of Quantum Chemistry 44 (1992), S. 235-250 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A pharmacophore for μ-opioid receptor recognition based on a study of the fentanyl class of opioids has recently been characterized in our laboratories. To validate this pharmacophore, we have extended our theoretical studies to include four opiate analogs from structurally different classes and with high affinity but varying selectivity for the μ-opiate receptor. An extensive conformational search of the flexible regions of these compounds has been carried out at two levels of approximations, using the CHARMm force field and the semiempirical molecular orbital method AM1. In a subsequent step, we have determined a series of structural, environmental, and electronic properties for each low-energy conformer of the analogs studied. All four analogs studied can assume a low-energy conformation in which at least three of the four stereoelectronic properties identified as modulators of recognition in the fentanyls are present in the same spatial arrangement. These results provide additional evidence for the plausibility of the proposed pharmacophore for μ-opioid receptor recognition. © 1992 John Wiley & Sons, Inc.
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  • 50
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    International Journal of Quantum Chemistry 44 (1992), S. 251-261 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A quantum chemical study of the two low-lying quartet states of seven model compound I iron-porphyrin complexes with varying axial ligands has been carried out using the INDO method. The varying axial ligands included in this study are five that are models for those in the intact enzymes: imidazole and imidazolate (model peroxidase HRP and CCP), CH3CONH2 (Gln175 mutant of CCP), PhO-1 (catalase), CH3S-1 (P450), and two that have been used in biomimetics of these enzymes: Cl-1 (hemin) and PhS-1 (model P450s). The purpose of these studies was to determine the role of the axial ligands in determining (i) the relative energies of the two nearly degenerate quartet electronic states of compound I, involved either as an a1u or a2u porphyrin π cation radical and (ii) the electron and spin distributions in the a1u and a2u radical cations of compound I. For most of the model complexes, including both HRP-I and CAT-I, a moderate effect of the axial ligand on the relative energy of these two states was observed and the a1u radical cation was found to be the ground state. The energy order of these two radical cations, however, was reversed in the P450-I model complexes, indicating an association of the unique property of the Fe=O bond breaking with an a2u radical cation. The symmetry-allowed overlap between the Fe=O and 3a2u orbitals may lower the activation energy for the Fe=O bond cleavage in P450-I. However, the calculated electronic and spin properties, including the unpaired spin and net charge on the oxygen and the Fe=O bond overlap density, important determinants of the reactivity of this complex in the ligand-Fe=O region, are very similar for all complexes and in both cation states. © 1992 John Wiley & Sons, Inc.
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  • 51
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    International Journal of Quantum Chemistry 43 (1992), S. 301-309 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The ground states of atoms and molecules Li-, Be, LiH, LiH2-, and Li2 have been calculated using the n-electron wave functions built up with two kinds of geminals. As a comparison, the above systems have been calculated with the Hartree-Fock self-consistent field and the multi-configuration self-consistent field method as well. The results show that the wave functions in this work are capable of describing the electron correlations, and both kinds of geminals can be taken as a starting point in building up n-electron ground states.
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  • 52
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    International Journal of Quantum Chemistry 43 (1992), S. 321-326 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio Hartree-Fock crystal orbital results were used as input for the calculation of exciton energies in the Tamm-Dancoff and random-phase approximation for polymers with helical symmetry. The calculations were applied to a hydrogen fluoride chain. We show that the Tamm-Dancoff method is a good approximation to the random-phase theory. Furthermore, the influence of the band-band interaction in the exciton calculation has been investigated.
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  • 53
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    International Journal of Quantum Chemistry 43 (1992), S. 353-374 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Following the recent developments of the charge sensitivity analysis (CSA) in the atoms-in-molecules (AIM) resolution, the corresponding CSA quantities in the orbital (or shell) resolution (OR) are defined. The OR electron population variables, in the ordinary closed-shell SCF problem, are the elements of the bond-order matrix P, and their conjugates, “chemical potentials,” FT = ∂E/∂P, are the respective Fock matrix elements, appropriate for the representation in question; here E is the SCF energy. The second derivatives ∂2E/∂P∂P define the OR hardness tensor from which all related OR CSs, e.g., the hardness, softnesses, Fukui function (FF) indices, etc., can be determined. The rigid potentials and hardness tensor, corresponding to the “frozen” orbital approximation, are examined in more detail, and the decoupled representation of the normal orbitals (NoO) is introduced, in which the rigid hardness tensor becomes diagonal. Illustrative valence-shell NoO contours for the water molecule are given and discussed. The new approximation for the OR FF indices, as the orbital occupation probabilities, is proposed on the basis of the density matrix functional development of Donnely and Parr for natural orbitals, and the relevant expressions for the molecular fragment (collection of orbitals) quantities are summarized.
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  • 54
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An exact solution for the electron-vibrational problem of the nonadiabatic molecular system has been obtained. By the quasi-particle transformation technique, the fermionic Hamiltonian has been derived and solved at the ab initio level. Results clearly and unambiguously show that the gap formation due to nonadiabatic electron-phonon coupling is mediated by the one-particle electron-phonon interaction term, whereas the two-particle one represents just a correction to the correlation energy. The temperature dependence of the gap and electronic specific heat connected with the electron-phonon coupling have also been derived.
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  • 55
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 56
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    International Journal of Quantum Chemistry 43 (1992), S. 713-723 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An exact vibration-rotation kinetic energy operator for polyatomic molecules has been obtained on the basis of Sutcliffe's method, in terms of curvilinear internal coordinates and rotational angular moment operators. This operator is derived from the kinetic energy operator in Cartesian coordinates by the successive transformations using the chain rule. This kinetic energy operator can be used not only for the system of any triatomic and tetraatomic molecules and common polyatomic molecules in chemistry, but also for the investigation of the collision problems between two molecules after some modifications. Finally, using this Hamiltonian, the rotation-vibration coupling equations of polyatomic molecules have been derived and discussed. © 1992 John Wiley & Sons, Inc.
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  • 57
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    International Journal of Quantum Chemistry 44 (1992) 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 58
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    International Journal of Quantum Chemistry 43 (1992), S. 747-753 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Using the BCH theorem, we express the Hamiltonian of a Morse oscillator as a complete series of powers of the creation and annihilation operators for the harmonic oscillator. In this way, we improve the results of a previous work that uses a Bogoliubov-Tyablikov tranformation to calculate the Franck-Condon factors by means of equivalent harmonic oscillators potentials. © 1992 John Wiley & Sons, Inc.
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  • 59
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    International Journal of Quantum Chemistry 43 (1992), S. 769-782 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The concept of a self-consistent field is developed within the version of density functional theory based on local-scaling transformations. It is shown that in this context there arise two types of consistency: one relating to the charge-consistency within an orbit and another to “orbit jumping.” The latter is analyzed in terms of one-particle equations. The connection with other methods is discussed. © 1992 John Wiley & Sons, Inc.
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  • 60
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The usefulness and reliability of the recent BSSE-free SCF algorithm based on the “chemical Hamiltonian approach” (CHA/F) is demonstrated by calculating potential curves for several hydrogen-bonded complexes with 4-31G, 6-31G, and 6-31G** basis sets. It is concluded that the CHA/F scheme gives results that are numerically close to those of the Boys-Bernardi a posteriori correction scheme but are free from the “overcompensation” characteristic of the latter at smaller distances and given basis sets. © 1992 John Wiley & Sons, Inc.
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  • 61
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    International Journal of Quantum Chemistry 44 (1992), S. 59-65 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Accurate lower bounds for the nonrelativistic lowest triplet states of 4He and ∞He and the He isoelectronic series up to Z = 10 were calculated by the method of variance minimization using wave functions with logarithmic terms. Furthermore, the method of Lehmann/Goerisch was extended to self-adjoint operators with an essential spectrum. Both methods were compared for 4He. © 1992 John Wiley & Sons, Inc.
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  • 62
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    International Journal of Quantum Chemistry 44 (1992) 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 63
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    International Journal of Quantum Chemistry 44 (1992), S. 301-313 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The calculations for biuret (B) and its two sulfur analogs, thiobiuret (TB) and dithiobiuret (DTB), were carried out using the ab initio LCAO-MO method at the Hartree-Fock level. The molecular structure of B was fully optimized using standard 3-21G and 6-31G* basis sets, while geometry optimizations of the TB's and DTB's were performed at the 3-21G* and 6-31G* levels. Only trans-isomers were found to be minimum structures on the HF/3-21G* potential-energy surfaces, whereas cis-forms are the first-order transition structures. The relative energies of the cis-conformers at MP 2/6-31G**//HF/6-31G* + 0.9 zero point energy (ZPE) level are equal to 47.4, 51.7, and 59.7 kJ mol-1 for TB, B, and DTB, respectively. Because of relatively large dipole moments, cis-conformers are predicted to be stabilized in polar solvents.
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  • 64
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    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: This work examines the possibility of finding an electron-hole and an electron-pair simultaneously in a π-system substituted by an electron-donating (NH2) and/or electron-withdrawing (NO2) group. The contributions of various ionic [(+), ( ↑ ↓ )] structures are calculated from ab initio SCF-CI wave functions, using a recently developed general multielectron population analysis. The molecules studied are two monosubstituted ethylenes, aminoethylene and nitroethylene, and a disubstituted ethylene, the 2-nitroethenamine (push-pull ethylene) in its two configurational forms. The influence of the NH2 and/or NO2 group in delocalization and ionic (vs. covalent) character of the C=C double bond are investigated, along with examining the experimental chemist formalism of electron-pair “displacements” in several resonance structures. Analysis of mutual dependence of an electron-hole and an electron-pair, at short and long distances, leads to the conclusion that a push-pull π-system can stimulate the simultaneous existence of an electronhole and an electron-pair even for nonvicinal positions. The relationship between the electronpair distributions and contributions of the corresponding structures are also examined; the effects of electronic correlation are analyzed as well. © 1992 John Wiley & Sons, Inc.
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  • 65
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    International Journal of Quantum Chemistry 44 (1992), S. 393-404 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A systematic procedure has been developed to construct an electronic energy matrix for diatomics in the basis of antisymmetrized products of atomic wave functions represented as linear combinations of coupled momenta eigenfunctions. The exchange matrix element is expanded in powers of electronic interchange between atoms. General expressions of many-electron angular coefficients have been obtained for all types of products of one- and two-electron and overlap integrals in energy matrix elements. © 1992 John Wiley & Sons, Inc.
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  • 66
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    International Journal of Quantum Chemistry 44 (1992), S. 405-411 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The one-range addition theorem [J. Fernández Rico, R. López, and G. Ramírez, Int. J. Quantum Chem. 34, 121 (1988)] for the Yukawa potential is rederived in a very simple manner using weakly convergent expansions [E. J. Weniger, J. Math. Phys. 26, 276 (1985)] of plane waves in the Fourier representation. It is discussed in which sense the addition theorem converges. The relation to Sturmian expansions of Coulomb Green functions is pointed out. © 1992 John Wiley & Sons, Inc.
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  • 67
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    International Journal of Quantum Chemistry 44 (1992), S. 691-698 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Both ab initio and semiempirical electronic structure calculations are used to investigate the molecular and electronic structures and eneregetic stabilities of an unusual bridged compound with the general formula [Y—SiH3—X—SiH3—Y]-, with Y = H or F and X = H, CH3, NH2, OH, F, or Cl. Most of these bridged anions are quite stable relative to YSiH3 + XSiH3Y-, and the stability is predicted to increase considerably when Y = H is replaced with Y = F.
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  • 68
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    International Journal of Quantum Chemistry 44 (1992), S. 229-241 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An algorithm is presented which produces localized cluster region orbitals which may be useful in embedded cluster calculations. The orbital occupation number matrix, n, for a trial set of functions is evaluated using the density matrix computed from an extended system. Surprisingly, when using Hartree-Fock (HF) orbitals from finite cluster calculations, several partially occupied orbitals having nii different from 2 and 0 are found. The localization is accomplished by applying Jacobi transformation to n, mixing surrounding functions with the cluster region, so that the final set of localized functions have ñii with either 2 or 0. Numerical examples of the algorithm are given for Li clusters simulating the Li(100) surface. © 1992 John Wiley & Sons, Inc.
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  • 69
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    International Journal of Quantum Chemistry 44 (1992), S. 243-252 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Improvements in MO programs and computer speeds have dramatically increased the size of molecules that can be optimized by MO methods. This has put a considerable strain on existing geometry optimization techniques. For a given molecular system, the performance of a geometry optimization method can vary significantly depending on the coordinates used. Nonredundant internal coordinates (e.g., Z-matrix coordinates) are easily constructed and behave well for acyclic molecules but can be quite troublesome for cyclic systems. Cartesian coordinates behave well for rigid, cyclic molecules but can be quite poor for flexible, acyclic systems. Mixed cartesian and internal coordinates have been suggested as a means of combining the best of both approaches and are particularly convenient for cyclic systems with flexible substituents. The efficiency of these different approaches is compared for a number of molecules. © 1992 John Wiley & Sons, Inc.
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  • 70
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    International Journal of Quantum Chemistry 44 (1992), S. 265-270 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The Löwdin α-function method, augmented by the computer-generated C matrix, is applied to the H2+ molecule. A LCAO (linear combination of atomic orbitals) method is employed using several s-orbitals with equal screening constants. Precautions are taken to avoid cancellation errors, thereby achieving convergence with increasingly higher orbitals. © 1992 John Wiley & Sons, Inc.
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  • 71
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    International Journal of Quantum Chemistry 44 (1992), S. 291-298 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Simple analytic approximations to the moments of electronic momentum 〈pk〉 (k = -2, -1, 1, 2, 3, 4) of the neutral atoms from hydrogen through uranium are presented. These approximations are generated by using Thomas-Fermi-Dirac-Scott and hydrogenic results to guess suitable functional forms, and then fitting the latter to tabulated Hartree-Fock (HF) moments. The root mean square (rms) percent errors of our best functions for 〈pk〉 with k 〉 1 are less than 0.6%. The best functions for 〈p〉 and 〈p-1〉 have a rms percent error of less than 2%. The 〈p-2〉 moments exhibit very strong shell structure, and our best function has a rms percent error as large as12%. © 1992 John Wiley & Sons, Inc.
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  • 72
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    International Journal of Quantum Chemistry 44 (1992), S. 311-318 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We use the continuous formulation for the matrix Hartree-Fock method, called the generator-coordinate-Hartree-Fock method, to rephrase a theorem of Nakatsuji et al. concerning the improvement of the Hellmann-Feynman forces calculated with nuclei-centered GTO basis functions. We show that we do not need to increase the size of the basis set in order to obtain reliable Hellmann-Feynman forces, but just use a self-consistent set so that, starting with some s-type GTOS, the p-type GTOS are a subset of the derivatives of those s-type GTOS, and so on. We illustrate this feature in calculations on small dyatomic molecules. © 1992 John Wiley & Sons, Inc.
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  • 73
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    International Journal of Quantum Chemistry 44 (1992), S. 319-331 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We explore a hybrid methodology in which the Hartree-Fock energy and density are combined with the nonlocal exchange functional of Becke (1988) and the nonlocal correlation functional of Lee, Yang and Parr (1988) to yield an estimate of the Schrödinger total energy. The resulting mean absolute deviation from a large number of experimental atomization energies, ionization potentials, electron affinities, and proton affinities is 3.86 kcal/mol. Performance comparisons are made with the recently developed G2 theory and nonconventional interpretations of the density functionals are discussed. © 1992 John Wiley & Sons, Inc.
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  • 74
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    International Journal of Quantum Chemistry 44 (1992), S. 347-358 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The nonlocal weighted density approximation (WDA) to the exchange and kinetic-energy functionals of many electron systems proposed several years ago by Alonso and Girifalco is used to compute, within the framework of density functional theory, the ground-state electronic density and total energy of noble gas atoms and of neutral jellium-like sodium clusters containing up to 500 atoms. These results are compared with analogous calculations using the well known Thomas-Fermi-Weizsäcker-Dirac (TFWD) approximations for the kinetic (TFW) and exchange (D) energy density functionals. An outstanding improvement of the total and exchange energies, of the density at the nucleus and of the 〈r-1〉 expectation values is obtained for atoms within the WDA scheme. For sodium clusters we notice a sizeable contribution of the nonlocal effects to the total energy and to the density profiles. In the limit of very large clusters these effects should affect the surface energy of the bulk metal. © 1992 John Wiley & Sons, Inc.
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  • 75
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    International Journal of Quantum Chemistry 44 (1992), S. 1-14 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Kinetic studies of the hydrolysis of benzo[a]pyrene diol epoxides have shown that hydrolysis proceeds through an acid-catalyzed SN1 mechanism and that the rate-determining step is the formation of a benzylic carbocation. The formation of this carbocation indicates that the diol epoxide can react with nucleophilic sites on DNA as an alkylating agent. Previous work has indicated that the kinetic data could be accurately fit to kobsd = kH [H+] + k0, in which kH [H+] is the pseudo first-order rate constant for acid-catalyzed hydrolysis and k0 is the rate constant for spontaneous hydrolysis. The observation made in several laboratories, that DNA catalyzes the hydrolysis of carcinogenic diol epoxides, is of direct interest in light of our recent calculation indicating that the local environment of DNA has a high concentration of hydrogen ions. The significance of acidic domains around nucleic acids is primarily in the ability of the protons to catalyze reactions. We have hypothesized that the high concentration of hydrogen ions at the DNA surface is responsible for the catalysis and that the generation of benzylic carbocations at the DNA surface can result in either nontoxic tetraols or in mutagenic nucleotide adducts, depending on the nucleophile with which the carbocation reacts. The calculations presented here are concerned with qualitatively understanding the rate constants in terms of the mechanisms of the acid-catalyzed and the spontaneous (H2O-catalyzed) hydrolyses. © 1992 John Wiley & Sons, Inc.
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  • 76
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    International Journal of Quantum Chemistry 44 (1992), S. 517-526 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A multireference Møller-Plesset (MR-MP) perturbation method, at the second-order level, is applied to the potential energy curve of the ground state of N2, for comparison with a variety of standard ab initio methods. In spite of the drastic simplification, the MR-MP results are very reliable. The energy errors are almost independent of geometry, allowing unbiased treatment of potential energy curves. The potential efficiency and accuracy of the MR-MP approach are emphasized. © 1992 John Wiley & Sons, Inc.
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  • 77
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    International Journal of Quantum Chemistry 44 (1992), S. 527-541 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A detailed investigation has been made of the methodological dependence of computed structures, binding energies, and some vibrational properties of selected hydrogen-bonded complexes. Structures and vibrational frequencies have been computed using four different levels of theory: HF/6-31G(d), HF/6-31+G(d, p), MP2/6-31G(d), and MP2/6-31+G(d, p). Correlation energy effects on computed binding energies and the basis set dependence of these energies have been evaluated using the Dunning correlation-consistent polarized valence double-, triple-, and truncated quadruple-split basis sets (cc-pVXZ, with X = D for double, T for triple, and Q́ for truncated quadruple), and these same basis sets augmented with diffuse functions on nonhydrogen atoms (cc-pVXZ+). The results of these studies suggest that the methodology of MBPT(4) = MP4/cc-pVTZ+ electronic energies without the counterpoise correction computed at MP2/6-31+G(d, p) geometries is an appropriate theoretical model for investigating complexes RCN ⃛ HX, with R = H or CH3 and X = F or Cl. This model has been applied to investigate the structures, binding energies at absolute zero, binding enthalpies at room temperature, and vibrational frequencies of these complexes. Trends are noted, and comparisons are made with available experimental data. © 1992 John Wiley & Sons, Inc.
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  • 78
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    International Journal of Quantum Chemistry 44 (1992), S. 551-561 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We present an INDO/S molecular-orbital investigation of organic molecules containing a barrelene moiety that provides a rigid link between an aromatic donor and a maleic ester acceptor group. Molecules of this type have recently been synthesized and characterized spectroscopically. We discuss the ground state and various excited states both in vacuo and in solution. Solvent effects are incorporated by use of an electrostatic cavity model which is not restricted to a spherical cavity, but allows for a cavity shape that is adapted to the solute molecule. The calculations indicate low-lying charge-transfer (CT) excitations in the region of the first aromatic transitions, even in the gas phase. © 1992 John Wiley & Sons, Inc.
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  • 79
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    International Journal of Quantum Chemistry 44 (1992), S. 587-604 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The interaction of CO with rhodium clusters of different sizes is studied by means of Intermediate Neglect of Differential Orbital (INDO) calculations. Interactions of both σ and π type are well-known to contribute to the metal —CO bond formation. The relative importance of the repulsive Rh—5σCO interaction with the metal and the back-bonding to the π* CO empty orbital in the stabilization of the bond is analyzed as a function of the cluster size and the coordination geometry. It is found that the shift of charge in the larger metal clusters brings about a reduction of the Rh—5σ CO repulsion, and this redistribution is important to change the optimal CO bonding geometry from bridge to on top. © 1992 John Wiley & Sons, Inc.
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  • 80
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    International Journal of Quantum Chemistry 44 (1992), S. 761-771 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In this article we generalize the necessary conditions for a topological definition of the concept of molecular structure, showing that it may be independent of the particular criterion chosen, as long as it fulfills some general conditions and may give rise to the concept of a category of stable chemical structures. We proceed to introduce a local metric in the objects of this category by using the properties of metric space bundles, and this allows us to propose a general system of reaction mechanisms within this framework of the theory of categories. We conclude by proposing a single diagram that encompasses the whole theory of stable structures and reaction mechanisms. © 1992 John Wiley & Sons, Inc.
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  • 81
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    International Journal of Quantum Chemistry 44 (1992), S. 781-791 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The C2v, potential energy curves for the interaction of Ga, Ga+, and Ga2+ with the hydrogen molecule were calculated using the pseudopotential method of Durand et al. and triple-ζ gaussian basis sets. Electronic correlation has been taken into account by MRCI + MP2 calculations. We studied the 2B2(4p), 2A1(4p), 2A1(5s), and 2B2(5p) surfaces for the Ga + H2, the 1A1, 3B2, and 1B2 surfaces for Ga+ + H2 and the 2A1,(4s), 2B2, and 2A1(5s) surfaces for the Ga2+ + H2 reaction. We first analyzed the unrelaxed H2 approach to the metallic center, followed by the angle relaxation. The reactivity in all the interactions studied are analyzed with a model mechanism, previously proposed by us. © 1992 John Wiley & Sons, Inc.
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  • 82
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    International Journal of Quantum Chemistry 44 (1992), S. 271-290 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We present results for the total energy, the first ionization potential, and the electron affinity of the Li, Be, B, C, N, O, F, Mg, Si, Ca, Fe, and Ge atoms using the coupled-cluster method with several standard basis sets, including correlation consistent, polarized, and atomic natural orbital bases. We also consider multiplet splittings in Fe and the activation barriers of the reactions, H3 → H2 + H and FH2 → F + H2. These readily generated results are provided for comparison with Quantum Monte Carlo values. © 1992 John Wiley & Sons, Inc.
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  • 83
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    International Journal of Quantum Chemistry 44 (1992), S. 837-852 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The implementation of the density-functional scheme requires the knowledge of the exchange and correlation potential, Vxc, as a functional of the electron density. In the local-density approximation (LDA) this potential becomes a function of the local value of the density. This ansatz breaks down qualitatively at a surface because of its neglect of long-range electron-electron correlations in the presence of strong charge inhomogeneity. This breakdown is of relevance in the context of various surface spectroscopies. We outline a scheme for going beyond the LDA without invoking gradient expansions. This scheme is based on establishing an interrelation between density-functional theory and many-body perturbation theory. In this scheme Vxc is obtainable from the knowledge of the electron self-energy Σxc. We solve an exact integral equation relating these two quantities for the electron-gas surface with use of the GW approximation for the self-energy. We establish a “nonlocal” relation between Vxc and the electron density which allows us to carry out nonlocal density-functional calculations with the same ease as LDA-based calculations. We present results of the first application of our method for the case of Al and Pd surfaces. In addition, we report on work in progress devoted to a detailed comparison of the density-functional and quasiparticle pictures of electronic excitations at a metal surface. © 1992 John Wiley & Sons, Inc.
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  • 84
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    International Journal of Quantum Chemistry 44 (1992), S. 435-441 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We study the multiphoton absorption of the diatomic HF by time propagating a wave packet on the electronic potential under the action of a monochromatic laser field. We apply the pseudo-spectral splitoperator method for the propagation. We use different approximations for the potential and for the dipole function. We find that the quantitative details of the absorption spectrum are sensitive to the form of the dipole function. © 1992 John Wiley & Sons, Inc.
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  • 85
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    International Journal of Quantum Chemistry 44 (1992), S. 443-453 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The He I and He II photoelectron (PE) spectra of methyl pseudohalides (MeNCO, MeNCS, MeNCSe, MeSCN, and MeSeCN) have been recorded. Assignments are based on ab initio quantum chemical calculations, and their comparison with high-resolution He I spectra and He I/He II band intensity ratios. Contradictions found in previous works are largely resolved. The strikingly different spectra of the isomeric forms are interpreted using a consistent model for a methyl-distorted NCX substructure. The effects of the heavy atoms are also discussed. © 1992 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 44 (1992), S. 673-686 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We report on the progress in a study to determine the structure of radiation-induced Mn defect complexes in CaF2. The purpose of this study is to correlate the calculated transition energies with those measured experimentally and to determine what effect the inclusion of the external lattice field has on these transitions. Unrestricted open shell Hartree-Fock Self-Consistent Field (UHF) calculations were performed on the unperturbed F-center [Ca4F6Vac]1+ S = 1/2, the unperturbed Mn2+ center [MnF8]6- S = 5/2, S = 3/2 and the Mn2+ perturbed F-center [Mn2Ca2F6Vac]1+ S = 11/2, S = 9/2 defect clusters. A method by which the effects of the external lattice field may be incorporated in the SCF procedure is developed and used in these calculations. Several transition energies are calculated and Mulliken population results for these clusters are presented and discussed. © 1992 John Wiley & Sons, Inc.
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  • 87
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    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 717-724 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The conventional use of pure angular momentum state representations in the algorithms for reactive scattering is discussed and the merits of alternative formulations in a finite element framework are considered with respect to possible numerical effectiveness. Four- and seven-dimensional hyperspheres are discretized as means of describing three- and six-dimensional scattering problems. © 1992 John Wiley & Sons, Inc.
    Additional Material: 1 Ill.
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  • 88
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 725-736 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Surface-molecule interactions and reactions are important elementary steps of catalytic reactions. Since they involve interactions between infinite and finite systems, modelings are necessary for theoretical investigations of catalytic reactions on a surface. The cluster model (CM) is most frequently used for quantum chemical calculations but neglects the effect of the bulk solid. For including such effect, ( 1 ) embedding the cluster onto a surface (actually into a larger cluster) is a method proposed by Grimley and Pisani, and (2) dipping the adcluster (admolecule + cluster) onto the electron bath of the solid and letting the system be at equilibrium for electron and spin exchanges is another model proposed by Nakatsuji. We show some applications of the embedding cluster model (ECM) and the dipped adcluster model (DAM). We will also report in the lecture on the study of the photochemical decomposition reaction of MnO4- into MnO2- + O2. © 1992 John Wiley & Sons, Inc.
    Additional Material: 10 Ill.
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  • 89
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    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 751-760 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: External field effects are introduced in both charge and orbital control terms of Klopman-Salem formalism for the study of chemical reactivity in condensed phase. An analytical expression is derived for the change of the interaction energy between a nucleophile and an electrophile from gas to solution phase. The resulting simple expression contains the effect of the external field, in terms of the variation of the electrophilic superdelocalizability index associated with the highest occupied molecular orbital (HOMO) of the nucleophile. Two classical reactions are analyzed to illustrate the usefulness and reliability of the proposed formalism. © 1992 John Wiley & Sons, Inc.
    Additional Material: 3 Ill.
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  • 90
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    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 773-779 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: There is a very simple criterion showing whether two (or more) Hamiltonians describe strictly additive effects. Based on this criterion, an attempt is made to develop techniques that would permit treatment of different not strictly additive interactions by separating out their components, which can be considered strictly additive, and describing the nonadditive (interference) effects as some perturbation. As a first result, a promising scheme has been developed for estimating matrix eigenvalues by an ∼ N2 procedure. © 1992 John Wiley & Sons, Inc.
    Additional Material: 4 Ill.
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  • 91
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    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 793-806 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An ab initio analysis of the reaction of molecular hydrogen with the Zr-imido complex (NH2)2Zr= NH is reported. Several interesting points are noted. The calculated stretching frequency of the Zr=N bond in (NH2)2Zr=NH is 860 cm-1 when properly scaled to account for electron correlation effects. The value supports the assignment of an infrared (IR) band at 865 cm-1 to the Zr=N stretch of a tetrahydrofuran (THF) adduct of the putative reactive intermediate (NHSi′)2Zr=NSi′. Although a weakly bound, H2 complex is found, the interaction is small (1.3 kcal mol-1) compared with experimentally characterized H2 complexes. Variations in bond lengths and intrinsic stretching frequencies demonstrate that π-bonding for amido (NH2) ligands can be substantial in a coordinatively saturated complex. H2 activation by the bis(amido)imido reactive intermediate is calculated to be significantly more favorable by the 1,2-addition of H2 across the Zr=N bond to form the tris(amido)hydride than a sigma-bond metathesis pathway. The transition state (TS) for the addition of H2 across the Zr=N bond of the bis(amido )imido complex is 9.8 kcal mol-1 above the charge transfer (CT) complex, (NH2)2Zr=NH = H2 at the MP2 level; the reverse process, extrusion of H2 from the tris(amido)hydride has a 28.2 kcal mol-1 barrier. The geometry of the four-center TS is of interest, deviating markedly from that of a square and being more “kite” shaped (i.e., one obtuse and three acute angles). Mulliken Bond Overlap Populations suggest that there is some interaction between the Zr and the H atom being transferred (Hl) in the various TSs studied. It is suggested that the interaction plays an important role in the ability of the Zrimido complexes, and indeed other high-valent, multiply bonded complexes, to activate X=H (X=H, C, Si, N, and H) bonds. The design of materials and catalysis precursors which enhance the metalhydrogen interaction in the TS could conceivably lead to lower chemical vapor deposition (CVD) processing temperatures and higher catalytic activities. © 1992 John Wiley & Sons, Inc.
    Additional Material: 3 Ill.
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  • 92
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 817-835 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The usefulness of various combinations of MCSCF and CI methods in computing correlated proton transfer potentials is investigated for the systems, HF2-, H7N2+, H3O2-, and H5O2+. MCSCF calculations can accurately determine proton transfer barriers, provided the correlation is limited to the proton transfer process. The proper correlated space can be obtained more easily if the canonical occupied MOs are first subjected to a localization. Various means are tested of including additional electron correlation into the MCSCF methods. CIS and CISD calculations are performed following MCSCF expansion of the wave function using various different MCSCF reference wave functions. The MCSCF + CISD results are excellent, being fairly independent of choice of virtual Mos, although it is important that the occupied orbitals be balanced between the donor and acceptor. Localizing the occupied MOS prior to the MCSCF part of the calculation again results in a further improvement. These results are compared to CI computations using the canonical orbitals (and which are not preceded by MCSCF preparation of the wave function) and to Møller-Plesset results. © 1992 John Wiley & Sons, Inc.
    Additional Material: 8 Tab.
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  • 93
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 41 (1992), S. 163-185 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The statistical state space is discussed in terms of the geometry of normed real vector spaces with particular reference to the novel concept of direction distance. Specialization to the geometry of the measure cone and the correspondingly specialized concept of mixing distance suggest strong mutual relationship as is shown subsequently in physically required generality. Thereby, the phenomenon of irreversibility attains an interpretation that seems to be the canonical mathematical background for classical and nonclassical statistical physics.
    Additional Material: 1 Ill.
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  • 94
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 41 (1992), S. 187-211 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The freeon tensor product basis provides a rapid method for the evaluation of matrix elements in the unitary group formulation of quantum chemistry. The method employs fast transformations between the Gel'fand and freeon tensor product basis.
    Additional Material: 3 Ill.
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  • 95
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    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 41 (1992), S. 371-379 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An adiabatic reaction path for hydrogen abstraction from methane by methyl is computed by quantum chemical methods and then symmetrized by properly defining the reaction coordinate. The theoretical barriers are then fitted with the barriers defined by the parabolic and Eckart functions. Rate constants for the hydrogen and deuterium-abstraction processes via tunneling at low temperatures are then computed.
    Additional Material: 4 Ill.
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  • 96
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    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 41 (1992), S. 383-384 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Additional Material: 1 Tab.
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  • 97
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    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 41 (1992), S. 387-397 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We review a formalism introduced by Fix and Heiberger in 1972 for solving the generalized (or nonorthogonal) eigenvalue problem for ill-conditioned symmetric matrices and we discuss its application in quantum chemistry. A few examples dealing with the calculation of high Rydberg orbitals are presented.
    Additional Material: 3 Tab.
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  • 98
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The numerical properties of a one-dimensional integral representation [H. P. Trivedi and E. O. Steinborn, Phys. Rev. A 27, 670 (1983)] for the Fourier transform of a two-center product of certain exponential-type orbitals (ETOs), the B functions [E. Filter and E. O. Steinborn, Phys. Rev. A 18, 1 (1978)], are examined. These functions span the space of ETOs. Hence, molecular integrals for other ETOs, like the more common Slater-type orbitals, may be found as finite linear combinations of integrals with B functions. The main advantage of B functions is the simplicity of their Fourier transform that makes the derivation of relatively simple general formulas for molecular integrals with the Fourier transform method possible. The integrand of the integral representation mentioned above shows sharp peaks, making, in the cases of highly asymmetric charge distributions and/or large momentum vectors, usual quadrature methods rather slow. New quadrature schemes are presented that utilize Möbius-transformation-based quadrature rules. These rules are well suited for the numerical quadrature of functions possessing a sharp peak at or near one boundary of integration [H. H. H. Homeier and E. O. Steinborn, J. Comput. Phys. 87, 61 (1990)]. Numerical results are presented that illustrate the fact that the new quadrature schemes are much more efficient than are automatic quadrature routines.
    Additional Material: 3 Tab.
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  • 99
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 41 (1992), S. 557-579 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Potential energy surfaces can be classified into combinatiorial equivalence classes, based on their partitioning into catchment regions. Two classification theorems are proven: one for reaction spheres and another for reaction tori. A method for constructing all possible equivalence classes of reaction spheres and reaction tori is presented. As illustration of the general results, it is shown that not all the two-dimensional reaction spheres are combinatorially equivalent to polyhedra in the three-dimensional Euclidean space. As examples, several reaction spheres are calculated by using the RHF method at the 3-21G* level, describing the interactions between a series of polyatomic ions and H+. The calculations show that the potential energy surface of the CO32- …H+ interaction, combinatorially equivalent to that of the NO-3 …H+ interaction, is not combinatorially equivalent to any polyhedron in 3-space; however, the combinatorially different potential energy surface of the PO43- … H+ interaction is equivalent to a polyhedron in 3-space. The topological classification scheme is proposed for the study of similarities between various families of chemical reactions.
    Additional Material: 10 Ill.
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  • 100
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    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 41 (1992), S. 599-611 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The electronic influence of the matrix on several adsorption sites of the CO/Ni—Cu(110) system has been studied using a semiempirical molecular orbital calculation. A negative ligand effect of a copper matrix on monometallic nickel sites and a less important ligand effect of a nickel matrix on copper sites have been found and explained in base on the electronic structure. Bridge nickel-copper sites show an intermediate negative ligand effect within a Cu matrix. The results of the theoretical calculation are compared with the available experimental data.
    Additional Material: 5 Ill.
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